Microwave communication system for moving land vehicle

ABSTRACT

The microwave communication system, in its preferred form, includes a stationary waveguide through which microwave energy flows in surface waves in the fundamental mode. The waveguide preferably is a semicylindrical dielectric rod with a metallic conducting shield on the flat surface of the rod, and with the remainder of the rod unshielded. An antenna having the same general guide configuration is positioned near the waveguide to transmit and receive communications to and from the waveguide. The antenna either is stationary or is mounted on a vehicle such as a railroad car and positioned so as to always be near the waveguide. One form of antenna actually comprises a novel directional coupler, and another is a resonant antenna. In the waveguide fabrication method, the pattern of the lines of force of the electrical field in an unshielded waveguide of a given shape is determined, and a plane perpendicular to the lines is located. A waveguide is formed with the original shape but cut away along the perpendicular plane. Then a conductive shield is located at the resulting surface.

United States Patent Inventor A isi. No.

Filed Patented Assignee MICROWAVE COMMUNICATION SYSTEM FOR MOVING LANI) VEHICLE 24 (.lalmu, l6 Drawlng Figs.

OTHER REFERENCES D. D. King, Dielectric Image Line, Journal of Applied Physics, Vol. 23 June I952, pp. 699- 700 333-95l Primary Examiner-Herman Karl Saalbach Assistant Examiner- Paul L. Gensler AttarneyCurtis, Morris and Safford ABSTRACT: The microwave communication system, in its preferred form, includes a stationary waveguide through which microwave energy flows in surface waves in the funda' mental mode. The waveguide preferably is a semicylindrical dielectric rod with a metallic conducting shield on the flat surface of the rod, and with the remainder of the rod unshielded. An antenna having the same general guide configuration is positioned near the waveguide to transmit and receive communications to and from the waveguide. The antenna either is stationary or is mounted on a vehicle such as a railroad car and positioned so as to always be near the waveguide. One form of antenna actually comprises a novel directional coupler, and another is a resonant antenna. In the waveguide fabrication method, the pattern of the lines of force of the electrical field in an unshielded waveguide of a given shape is determined, and a plane perpendicular to the lines is located. A waveguide is formed with the original shape but cut away along the perpendicular plane. Then a conductive shield is located at the resulting surface.

PATENTEU strzs l97l SHEET 1 OF 4 INVENTO R: MAM m G. 45545 and fFTZNEYS.

MICROWAVE COMMUNICATION SYSTEM FOR MOVING LAND VEHICLE This invention relates to microwave electrical energy transmission systems, waveguides for such systems, and methods for fabricating such waveguides. More particularly, this invention relates to microwave communications systems for communication with a moving land vehicle such as a railroad car.

In the field of microwave communications, more efficient and effective means for coupling microwave energy from waveguides to receivers and transmitters long have been sought. This problem has become particularly troublesome in the development of a broadband communications system for communicating with moving land vehicles such as railroad cars; A relatively broadband communications system is required for communicating such information as television signals and the like to railroad cars.

Various microwave communications systems have been proposed for communicating with high'speed land vehicles. Several of such proposed systems are described in the Technical Report for the U.S. Department of Transportation, Office of High Speed Ground Transportation entitled Use of Surface Waves in Communicating with High Speed Vehicles dated June 5, 1968. That publication describes several dif ferent microwave systems for communicating with land vehicles, all of which have certain shortcomings. Most of the waveguides described for use in such prior systems are very expensive to fabricate and mount, and usually are adversely affected by environmental conditions such as snow, ice, birds, etc., and by the attachment of mounting brackets needed to support the waveguides.

In accordance with the foregoing, it is an object of the present invention to provide a simple and inexpensive waveguide structure which is relatively insensitive to environmental conditions and mounting structures, and which is relatively easy to use for the purpose of communication. Furthermore, it is an object of the present invention to provide a relatively compact antenna or coupling structure for use in communicating with the waveguide structure. Still further, it is an object of the invention to provide a simple and efficient method of fabricating such waveguide structures.

Further objects and advantages of the present invention will be apparent from the following description and drawings.

In the drawings:

FIG. 1 is a perspective view of a microwave communications system constructed in accordance with the present invention;

FIG. 2 is an elevation and partially schematic view of another embodiment of the communications system of the present invention;

FIG. 3 is a cross-sectional view taken along line 3-3 of FIG. 1;

FIG. 4 is a schematic diagram illustrating one of the principles of operation and the method of the present invention;

FIG. 5 is a perspective and partially schematic broken-away view of another embodiment of the present invention;

.FIG. 6 is an enlarged perspective view of a portion of the waveguide structure shown in FIGS. 1 through 5;

FIG. 7 is a cross-sectional view taken along line 77 of FIG. 6;

FIG. 8 is a cross-sectional view taken along line 8-8 of FIG.5; v

FIG. 9 is a cross-sectional view taken along line 9-9 of FIG. 5;

F IG 10 is a perspective view of another embodiment of the present invention;

FIGS. 11-14 are graphs illustrating some of the operating parameters of the system of the present invention;

FIG. 15 is a schematic cross-sectional view taken along line 15-15 of FIG. 3 showing the approximate electrical field pattern believed to exist in and around the waveguide of the present invention; and

FIG. 16 is a cross-sectional view similar to FIG. 3 showing the approximate electrical and magnetic field patterns be lieved to exist in and around the waveguide of the present invention.

The communications system 20 shown in FIG. I includes a stationary waveguide 22, a conventional source 26 of microwave energy coupled to the waveguide 22, an antenna or coupling device 24, together with a microwave transmitter 28 and receiver 30 adapted to be coupled to the antenna 24. The waveguide 22 is stationary and is supported above the ground by means of a plurality of support brackets 32 which are driven into the ground. The antenna 24 is positioned directly below the waveguide 22, as is shown most clearly in FIG. 3, and also is stationary. It is supported by means of support members 34 which also are driven into the ground.

THE WAVEGUIDE STRUCTURE The waveguide structure is illustrated most clearly in FIG. 3. The antenna or coupling device 24 includes a section of waveguide structure which is identical in shape to the waveguide 22.

The waveguide 22 includes a semicylindrical rod 36 of dielectric material such as polyethylene. A channel or U- shaped conductive shield 38 is provided. A waveguide with a similar shape has been described by D. D. King in Dielectric Image Line, Joumal of Applied Physics," Vol. 23, June I952, pp. 699,700 and Properties of Dielectric Image Lines, IRE Trans. on Microwave Theory," Tech. Vol. MTT -3, pp 7581, Mar. 1955. The shield 38 has side-flanges 40 and 42 extending perpendicularly from a web 39. The flat surface 48 of the semicylindrical rod 36 is secured to the web 39 and the rod is centrally located within the shield. The rod 36 is secured in place by adhesive or by snap-fitting it into a shallow groove (not shown) in the web 39. Extending in a direction opposite to that of flanges 40 and 42 is a central flange or rib 44 to which the support brackets 32 are secured.

The waveguide 22 preferably is positioned as is shown in FIGS. 1 and 3 with the flanges 40 and 42 extending downwardly, and the web 39 substantially horizontal. With this arrangement, snow, rain, birds and other naturally-occurring foreign objects or substances will be prevented from reaching the rod 36 and disrupting the transmission of microwaves.

Only a portion of the waveguide 22 is shown in FIG. I. It is to be understood that the communications waveguide 22 can be made as long or as short as desired. Preferably, the waveguide will be a number of miles long, with repeaters positioned along the line at intervals of several miles. Communication between the waveguide and transmitters and receivers 28 and 30 can be accomplished by means of antennas such as the antenna 24 positioned along the length of the waveguide 22. Alternatively, curved waveguide coupling devices which are physically connected to the waveguide structure, or other well-known means can be used to couple to the waveguide 22.

OPERATIONAL PRINCIPLES OF THE WAVEGUIDE A substantial portion of the surface of the waveguide rod 36 is unshielded. This is highly advantageous because it allows the flux lines of the electric field in the rod to extend outwardly from the rod 36 so that they may be intercepted by an antenna or coupling device positioned at any location along the length of the waveguide 22. Moreover, the waveguide structure of the present invention has the advantageous features of a cylindrical waveguide, but does not have some of the problems commonly encountered in such a waveguide. For example, because of the position and shape of the shield member 38, the electric and magnetic fields in the waveguide member 36 do not rotate about the longitudinal axis as they would in an ordinary cylindrical waveguide.

FIG. 4 illustrates the method used in fabricating the waveguide structure of the present invention. First, an elongated waveguide member with a symmetrical cross-sectional shape is selected. Such a member is, for example, a cylindrical rod with a cross-sectional shape such as that formed by the two semicircles 36 and 44 shown in FIG. 4. The upper half 44 of the rod is shown in dashed outline. The lower half 36 shown in solid lines corresponds, of course, to the semieylindrical rod 36 shown in FIGS. 1 and 3. A mode of propagation of the microwaves through the cylindrical rod is selected, and a plane is located to which the electrical flux lines in the rod are substantially perpendicular. In the case of the cylindrical rod shown in FIG. 4, with energy being transmitted in the fundamental mode, the electrical flux lines in the rod are represented by the lines 46. A plane which is perpendicular to these flux lines is represented by the line 48 which represents a diametric plane which passes through the central axis of the rod. Then, the actual waveguide rod 36 is formed as half of the cylinder shown in FIG. 4, with the surface at 48 being a flat plane surface. Then, the conductive shield 38 is secured onto the surface 48, with the web 39 of the shield being flat. Since the electric flux lines in the full cylindrical rod would have been perpendicular to plane 48, the location of a conductive member on surface 48 will not disruptthe normal pattern of the electric field in the waveguide member. The reason for this is that the electric flux lines in the waveguide extend perpendicular to any conductive surface forming a boundary for the waveguide. Since the flux lines already are perpendicular to the plane in which the conductor is to be located, location of the conductor in that plane will not disrupt the pattern of the lines.

In a relatively long waveguide, it is desired to transmit microwave energy in the surface mode; that is, a mode in which a surface wave propagates in the axial direction of the waveguide but does not propagate in the radial direction. The reason why operation in the surface mode is desired is because this minimizes the loss of energy along the length of the waveguide and, therefore, minimizes the number of repeater stations and the amount of power required in the communications system.

It can be shown that for a cylindrical dielectric rod having a dielectric constant whose real part is 6,-, a surface microwave mode will be propagated along the cylinder if the conditions of the following equation are satisfied:

tit 5 In which:

6, is the real part of the dielectric constant of the dielectric material;

c is the speed oflight; and

V,is the phase velocity of the microwave energy.

In order to minimize energy losses and signal distortion, it is preferred that only one mode be excited and propagated along the dielectric rod 36. It can be shown that the dominant or fundamental mode of the rod 36 does not exhibit a cutoff frequency. This is one reason why it is preferred that the dominant or fundamental mode be used. With operation in the fundamental mode, as the frequency of the microwave energy decreases, the phase velocity approaches as a limit the speed of light; as the frequency increases, the phase velocity decreases to the limit at whichV;=6/ Each higher order mode of operation of the waveguide exhibits a frequency below which the surface wave cannot be sustained by the dielectric rod, and the electromagnetic wave propagates along the longitudinal axis of the rod and also in a direction radial to the rod. Thus, if the operating frequency is selected to be sufficiently small compared to the lowest cutoff frequency, these undesired higher order modes, if excited, are rapidly radiated away in a radial direction.

The selected surface wave mode of operation is the hybrid mode l-IE in which both the electric and magnetic fields exhibit components in the axial direction of the waveguide. The approximate direction of the electrical field flux lines 46 is shown in FIG. 4. As is shown in FIG. 15, the electrical flux lines 46-extend outwardly from the unshielded lower portion of the semicylinder 36 in reentrant loop patterns which are repeated at half-wavelength intervals along the length of the rod 36. The patterns in successive half-wavelengths are, of course, of opposite polarity. As is shown in FIG. 16, the magnetic flux lines 47 are perpendicular to the electrical flux lines 46 and form similar half-wavelength patterns.

It is desired to select a value of the phase velocity V, which is close to the speed of light. That is, it is desirable to select a value of c/V, which is close to unity. One reason for this is that it results in the elimination of higher modes of operation, as has been explained above. Furthermore, the rate of decay of the electromagnetic field outside of the dielectric rod 36 increases rapidly as c/V, increases. Thus, if a large value of c/V, were selected, the signal strength in the antenna 24 would be considerably weaker than if a lower value of c/V, were selected. Also, attenuation and distortion effects increase as c/V, increases. On the other hand, if c/V, is reduced to a value too close to unity, the stability of the surface wave mode is impaired. Thus, even small imperfections in the waveguide construction in surrounding objects may cause a relatively large radiation loss. Consequently, a compromise should be reached between a high and low value for c/V, in the final selection of waveguide geometry, dielectric material, and operating frequency.

FIG. 11 of the drawings shows the experimentally observed relationship between the quantity c/V, and c/wr for a waveguide 22 having an approximate geometry such as that shown at the bottom of FIG. 12. FIG. 11 shows that c/V, approaches unity rapidly as the wavelength of the microwave energy being transmitted becomes large compared to the diameter of the dielectric rod 36. Thus, increasing the wavelength and/or decreasing the rod diameter rapidly reduces the value of c/V, towards unity.

FIG. 12 illustrates the effect of the distance Y between the opposed flanges 40 and 42 of the shield 38 upon the phase velocity V The depth of the flanges in the experiment performed was approximately 0.50 inches, and the diameter of the rod 36 was approximately 0.553 inches. It can be seen that a shield having a spacing Y of 2 inches or more had relatively little effect on the phase velocity.

The finite conductivity of the metallic shield and the losses in the dielectric material of the rod 36 are responsible for the attenuation of the electromagnetic wave which propagates along the dielectric rod. For small values of 1,5 and (c/V l the attenuation constant due the energy losses in the dielectric rod is given by the following equation:

(2) fiNL as) F( C V 1 In which:

is a function of wr /c which increases slowly with eur /c.

FIG. 13 is a graph in which the function B 8 is plotted against the quantity c/wr for a dielectric rod of polyethylene having a dielectric constant e r=2.2 I v Equation [2] shows that the attenuation constant decreases as c/V, approaches unity, and that for constant values of c/V, the attenuation constant 13,, decreases with increasing values of r Thus, for constant values of c/V,, the attenuation constant [3,, increases with the operating frequency.

It is preferred that the material of the rod 36 be a relatively inexpensive dielectric material such as polyethylene, but it can be one of the polytetrafluoride compounds such as Teflon" or it can be nylon, or a similar material. The shield 38 should be made of a material having good electrical conductivity, such as aluminum or copper.

. for the communications as well as attenuation and other factors. The operating frequency for this specific example was selected at around 9,000 megacycles per second. This operating frequency makes practical a bandwidth of from to I00 megacycles. It is preferred that a value of c/V,of less than 1.05 be used, and the most desirable range for c/V, appears to be from 1.01 to 1.02.

ALTERNATIVE WAVEGUIDE STRUCTURE An alternative waveguide structure 108 is shown in FIG. 10. In the structure 108, a slotted metallic conductive waveguide member 110 replaces the semicylindrical polyethylene member 36 shown in FIGS. 1 through 6. Member 110 is of rectangular cross section, with slots lll spaced a quarter wavelength apart, each being a quarter wavelength deep. The member 110 is secured to the web 39 of the shield 38.

The waveguide structure 108 operates in a manner similar to the waveguide structure previously described. The slotted member 110 slows the electromagnetic waves to a phase velocity less than the speed of light in air, and the electric flux lines in the member 110 are perpendicular to the web 39 of the shield 38. Electric flux lines extend outwardly from the surfaceof member 110, and they can be intercepted and received by an antenna having a similar structure, or one having the structure described above.

WAVEGUIDE TRANSITION AND CONNECTING STRUCTURES As it is shown in FIG. 1, the waveguide system includes a number of waveguide sections 22, each of which has a flange 50 at each end which is bolted to a similar flange on the next adjoining waveguide section. FIGS. 6 and 7 show that one of the flanges 50 has a vertical slot 52 connected with a horizontal slot 56 in the end surface 54 of the flange 50. The depth of each of the slots 52 and 56 is a quarter wavelength of the frequency being transmitted in the communications system. This L-shaped slot arrangement forms a conventional quarterwavelength choke which minimizes or eliminates leakage at the joints between the adjoining waveguide sections, and yet does not impair transmission of the microwaves along the waveguide.

Referring again to FIG. 1, the output of the conventional microwave source 26 (which may be a magnetron, klystron or the like) is fed through a rectangular waveguide 61 and another rectangular waveguide 60 to a transition coupling device 58. As is shown in FIGS. 8 and 9, the transition device 58 is a structure for providing a smooth transition of the waveguide from the semicylindrical form shown at 98 to the rectangular form of the waveguide 60. A polyethylene body having a semicylindrical portion 100 of gradually increasing diameter is connected to the semicylindrical portion 98. Portion 98 has the same diameter as the remainder of the semicylindrical member 36. The top portion 104 of the left end of'the polyethylene body is flattened so that the total vertical height of the body is about equal to the internal height of the rectangular waveguide. The sides 102 of the portion 100 are flattened for a distance. and then are tapered to a relatively sharp edge 106 which is positioned just short of the entrance to the rectangular waveguide 60. It has been found that increasing the diameter at 100 improves the matching properties with rectangular waveguides having small transverse dimensions.

THE RESONANT ANTENNA The antenna 24 shown in FIG. 1 is a resonant antenna. It consists of a section of waveguide structure 22 (see FIG. 3) having a length which is equal to an integral number of halfwavelengths of the operating frequency. Both ends of the waveguide rod are, in effect, short-circuited by means of conductive end plates 31 and 33. End plate 33 has a window through which the resonated electromagnetic energy intercepted by the rod 36 of the antenna 24 passes into a waveguide 27, to a detector or transducer device 35, and through another waveguide or shielded cable indicated at 29 to the receiver 30. Of course, a switching device is provided for switching either the transmitter 28 or the receiver 30 into connection with the line 29 to either transmit energy to or to receive energy from the waveguide system. The resonant antenna has an effective length which is considerably longer than its actual length.

THE DIRECTIONAL COUPLER DEVICE FIG. 5 shows an alternative form of the communications system which is identical to the system shown in FIG. 1 except that the antenna 24 is replaced by a directional coupler 61. The directional coupler 61 includes a length of waveguide structure 22, exactly the same as the structure described above, with a transition device 58 connected at each end. The electromagnetic waves which are intercepted by the dielectric rod 36 of the coupler 61 pass through a rectangular waveguide 60, a rectangular waveguide elbow 64 and into a detector device 65. Thence, the waves travel over a cable 67 to the receiver 30.

At the other end of the coupler 61 is the rectangular waveguide 66 which feeds the waves from waveguide 66 into a conventional absorber 68 whose impedance is matched to the impedance of the directional coupler and which receives and absorbs all reflected waves which travel into it. Unlike the communications system shown in FIG. 1, the coupler 61 receives and transmits only waves traveling in one direction; that is, only those waves traveling in the direction of the arrow 69 shown in FIG. 5. With this arrangement, the energy traveling in the stationary waveguide 22 creates electromagnetic waves whose flux lines extend downwardly from the rod 36 are are intercepted by the rod 36 in the directional coupler 61. The waves induced in the coupler travel from left to right in the rod 36 of the coupler and into the detecting system at the right end of the coupler. All reflected waves travel from right to left to the absorber 68 where they are absorbed so that they do not interfere with the reception of the desired communications signals.

It is evident from the foregoing that the coupling structure 61 shown in FIG. 5 constitutes a new and advantageous directional coupler, used as an antenna in the present invention, which has advantageous utility in and of itself.

The directional coupler 61, when used as an antenna, has an advantage over the resonant antenna 24 shown in FIG. 1, in that it is effective over a relatively larger bandwidth than is the resonant antenna 24. Also, the directional coupler accepts only those signals traveling in the left to right direction in the waveguide, and rejects all signals traveling in the opposite direction.

The power coupled from the waveguide 22 to the directional coupler 61 is a function of the distance x,,, between the stationary waveguide and the directional coupler, and of the lateral separation y between the stationary waveguide and the directional coupler. The directions of these quantities .r and y,,, are indicated in FIG. 3. FIG. 14 shows an experimentally obtained graph in which the coupled power P plotted in relative units, is plotted against the vertical separation x between the coupler and the stationary waveguide for several different values of y the lateral separation. The same structure was used in the tests whose results are plotted in FIGS. ll, 12, and 13. FIG. 14 shows that the power level decreases at increasingly higher positions on the graph. Thus, the coupled power decreases as the vertical separation 2: increases.

Both the resonant antenna 24 and the coupliilg antenna 61 have significant advantages beyond those already described when used in the communications system of the present invention. Both types of antennas can be used in a manner such that most spurious microwave energy in the atmosphere will automatically be rejected. Most of such spurious microwave energy has a value of c/V, which is less than one. Since the phase velocity of the microwave energy transmitted in the present communications system deliberately is made less than the speed of light in air, and the quantity c/V; is maintained at a value slightly greater than unity, over a significant number of wavelengths of the communicated energy, the spurious waves which might be picked up in the antenna or coupler and the transmitted wave will cancel one another. Since the present invention enables eflective and efficient transmission of communication at relatively high frequencies, e.g. 9,000 megacycles, the wavelength of the energy is relatively short, so that an antenna which is physically short (e. g. 3 or 4 feet) will be to 20 wavelengths long, and thus will provide an ample length for cancellation of the spurious waves. The antennas of the present invention do not need the special precision slots or holes required by some prior art coupling devices, and they conveniently and inexpensively use the same basic waveguide structure as the stationary waveguide 22.

Applicants communication system enables the transmission of communications at relatively high frequencies without an accompanying large amount of signal attenuation. Furthermore, and quite advantageously, the waveguide structure is relatively simple and inexpensive to construct; transmission is not adversely affected by the connection of mounting arms to the waveguide rib 44, or by birds, ice, snow or the like which might fall onto the shield 38 of the waveguide.

LAND VEHICLE COMMUNICATIONS SYSTEM F IG. 2 of the drawings illustrates the use of the communications system of the present invention to communicate with a moving land vehicle such as a railroad locomotive 70 traveling on rails 76 secured to railroad ties 78. The stationary waveguide 22 is mounted either on a series of relatively tall poles 72 alongside of the railroad tracks 76 so as to hand down close to the top of the locomotive 70, or is supported relatively near the ground by rods 32 of the type shown in FIG. 1.

if rods 32 of the type shown in FIG. 1 are used, the antenna 24 (which also can be a directional coupler such as coupler 61) is mounted on the axle 74 in the undercarriage of the locomotive by means of a bracket 80 and a suitable rotary coupling device which will prevent the antenna 24 from rotating with the locomotive axle or wheels. The bracket 80 can be connected to two successive axles 74 for lateral stability of the support, if necessary. As the antenna 24 travels along with the moving locomotive 70, it can both transmit and receive communications to and from the stationary waveguide 22 by means of a transmitter 28 or a receiver 30 coupled to the antenna 24 by means of a cable or waveguide system 82. By attaching the antenna 24 to the axles 74 of the locomotive, the amount of vertical movement of the antenna 24 with respect to the stationary waveguide 22 is minimized. Such undesired vertical movement can be further minimized by attaching the support rods 32 to the railroad ties 28 instead of the ground.

in the alternative arrangement in which the waveguide 22 is supported on tall poles 72, a coupler 61 is secured to the top of the locomotive 70 so that it is just underneath and slightly separated from the waveguide 22. It is to be understood, of course, that transmitters and receivers such as transmitter 28 and 30 are provided with this alternative embodiment, as they are with the embodiment first described.

The above description of the invention is intended to be illustrative and not limiting. Various changes or modifications in the embodiments described may occur to those skilled in the art and these can be made without departing from the spirit or scope of the invention.

I claim:

1. An antenna for a microwave transmission system, said antenna comprising a length of dielectric waveguide structure, a conductive shield on only a portion of the perimeter of said waveguide structure, said shield being substantially perpendicular to the lines of force of the electric field traveling in said waveguide structure in a predetermined mode, the length of said waveguide structure being equal to an integral number of half-wavelengths of the microwave energy being transmitted, and reflective termination means at each end of said antenna, one of said reflective terminations having a window providing an exit for microwave energy.

2. Apparatus as in claim 1 in which said waveguide structure is a semicylindrical rod.

3. Apparatus as in claim 1 in which said waveguide structure is a semicylindrical rod of dielectric material, and said shield has a web portion secured to the flat portion of said rod, and a pair of flanges extending from said web at positions spaced from said rod.

4. A system for communicating with a moving land vehicle, said system comprising, in combination, a vehicle capable of moving on a track on the ground, a stationary waveguide positioned along said track, said waveguide including semicylindrical dielectric waveguide means for guiding microwave elec tromagnetic energy at a phase velocity less than the speed of light, and a conductive shield on only a portion of the perimeter of said waveguide means, said shield extending along a diameter of the full cylinder of which said waveguide means in a part, an antenna mounted on said vehicle in a portion in which it is adjacent the unshielded portion of said waveguide to receive signals therefrom, and means for generating and sending said microwave energy through said waveguide in the HE mode.

5. Apparatus as in claim 4 in which said antenna is mounted adjacent the undercarriage of said vehicle and said stationary waveguide is located at a height so as to be closely adjacent said antenna when said vehicle is traveling on said track.

6. Apparatus as in claim 4 in which said stationary waveguide faces generally downwardly and said shield is above and covers said waveguide.

7. Apparatus as in claim 4 including a transmitter and receiver on said vehicle and adapted to be connected to said antenna.

8. Apparatus as in claim 4 in which said waveguide means includes a semicylindrical rod of dielectric material, said shield being positioned on the flat surface of said rod.

9. Apparatus as in claim 8 in which said shield is U-shaped with said rod being secured to the web between the arms of the U, and a plurality of supports connected to the shield on the outside of the U.

10. Apparatus as in claim 8 in which the guide portion of said antenna has substantially the same construction as the guide portion of said stationary waveguide.

11. In a system for communication with a movable body, elongated stationary semicylindrical dielectric waveguide means for guiding microwave electromagnetic energy at a phase velocity less than the speed of light in air, means for generating and sending said microwave energy through said waveguide means in the HE mode, a conductive shield on only a portion of the perimeter of said waveguide means, said shield extending along a diameter of the full cylinder of which said waveguide means is a part, a portion of said waveguide means being unshielded, microwave signal coupling means movably positioned adjacent the unshielded portion of said waveguide means and adapted to move in the longitudinal direction in spaced apart relationship to said stationary waveguide means, and to couple microwave signals to and from said stationary waveguide means.

12. Apparatus as in claim 11 including receiving and transmitting means connected to said coupling means.

13. A system as in claim 11 in which the coupling means is a length of waveguide structure having substantially the same shape as that of said stationary waveguide means.

14. A system as in claim 1 1 in which the ratio of the speed of light to said phase velocity is approximately from 1.01 to 1.05.

15. Apparatus as in claim 11 including a waveguide structure in said coupling means, said waveguide structure having an effective length which is more than 10 times as long as the wavelength of the microwave energy being transmitted.

16. Apparatus as in claim 11 in which said unshielded portion of said stationary waveguide means faces generally downwardly, with said shield covering said waveguide means.

17. Apparatus as in claim 16 including a plurality of support structures for supporting said waveguide means above the ground, and means for fastening said shield to said support structures.

18. Apparatus as in claim 11 in which said signal coupling means has a partially shielded waveguide structure substantially identical to that of said stationary waveguide means, and means secured to the shield of said coupling means to hold it in a position such that the unshielded portion of the antenna waveguide structure faces that of the first-named waveguide structure.

19. Apparatus as in claim 18 in which said waveguide structure in said coupling means has a microwave absorber at one end, and microwave detector means at the other end.

20. in a microwave communication system, a stationary microwave guide comprising a semicylindrical dielectric guide member with an electrically conductive member on the surface of said guide member extending along a diameter of the full cylinder of which said member is a part, means for generating and sending said microwave energy through said guide member in the HE mode, a coupler positioned to move approximately parallel to said stationary guide at a distance therefrom to couple microwave signals to and from said stationary guide, said coupler being a waveguide having substantially the same shape as said stationary waveguide.

21. A system as in claim 20 in which the waveguide of said coupler has an efi'ective length which is more than 10 times as long as the wavelength of the microwave energy being transmitted.

22. A system as in claim 20 in which said electrically conductive member comprises a shield which covers said dielectric guide member, and including support members fastened to said conductive member to support said guide above a support surface.

23. A system as in claim 22 in which said shield has, substantially, an inverted U-shape.

24. In a microwave energy transmission system, elongated waveguide means for guiding microwave electromagnetic energy at a phase velocity less than the speed of light in air, a conductive shield on only a portion of the perimeter of said waveguide means, said shield being substantially perpendicular to the lines of force of the electrical field in said waveguide means when conducting microwave energy in a predetermined mode, a plurality of support structures for supporting said waveguide means above the ground and means for fastening said shield to said support structures, antenna means adjacent the unshielded portion of said waveguide means, said antenna means having a partially shielded waveguide structure substantially identical to that of the transmission system, and means secured to the shield of said antenna means to hold it in a position such that the unshielded portion of the antenna waveguide structure faces that of the first-named waveguide structure, said waveguide structure of said antenna having a length which is an integral multiple of half-wavelengths of the energy being transmitted, a pair of microwave reflectors at the ends of the antenna with an aperture in one reflector, and means for guiding energy out of said antenna through said aperture. 

1. An antenna for a microwave transmission system, said antenna comprising a length of dielectric waveguide structure, a conductive shield on only a portion of the perimeter of said waveguide structure, said shield being substantially perpendicular to the lines of force of the electric field traveling in said waveguide structure in a predetermined mode, the length of said waveguide structure being equal to an integral number of half-wavelengths of the microwave energy being transmitted, and reflective termination means at each end of said antenna, one of said reflective terminations having a window providing an exit for microwave energy.
 2. Apparatus as in claim 1 in which said waveguide structure is a semicylindrical rod.
 3. Apparatus as in claim 1 in which said waveguide structure is a semicylindrical rod of dielectric material, and said shield has a web portion secured to the flat portion of said rod, and a pair of flanges extending from said web at positions spaced from said rod.
 4. A system for communicating with a moving land vehicle, said system comprising, in combination, a vehicle capable of moving on a track on the ground, a stationary waveguide positioned along said track, said waveguide including semicylindrical dielectric waveguide means for guiding microwave electromagnetic energy at a phase velocity less than the speed of light, and a conductive shield on only a portion of the perimeter of said waveguide means, said shield extending along a diameter of the full cylinder of which said waveguide means in a part, an antenna mounted on said vehicle in a portion in which it is adjacent the unshielded portion of said waveguide to receive signals therefrom, and means for generating and sending said microwave energy through said waveguide in the HE11 mode.
 5. Apparatus as in claim 4 in which said antenna is mounted adjacent the undercarriage of said vehicle and said stationary waveguide is located at a height so as to be closely adjacent said antenna when said vehicle is traveling on said track.
 6. Apparatus as in claim 4 in which said stationary waveguide faces generally downwardly and saiD shield is above and covers said waveguide.
 7. Apparatus as in claim 4 including a transmitter and receiver on said vehicle and adapted to be connected to said antenna.
 8. Apparatus as in claim 4 in which said waveguide means includes a semicylindrical rod of dielectric material, said shield being positioned on the flat surface of said rod.
 9. Apparatus as in claim 8 in which said shield is U-shaped with said rod being secured to the web between the arms of the U, and a plurality of supports connected to the shield on the outside of the U.
 10. Apparatus as in claim 8 in which the guide portion of said antenna has substantially the same construction as the guide portion of said stationary waveguide.
 11. In a system for communication with a movable body, elongated stationary semicylindrical dielectric waveguide means for guiding microwave electromagnetic energy at a phase velocity less than the speed of light in air, means for generating and sending said microwave energy through said waveguide means in the HE11 mode, a conductive shield on only a portion of the perimeter of said waveguide means, said shield extending along a diameter of the full cylinder of which said waveguide means is a part, a portion of said waveguide means being unshielded, microwave signal coupling means movably positioned adjacent the unshielded portion of said waveguide means and adapted to move in the longitudinal direction in spaced apart relationship to said stationary waveguide means, and to couple microwave signals to and from said stationary waveguide means.
 12. Apparatus as in claim 11 including receiving and transmitting means connected to said coupling means.
 13. A system as in claim 11 in which the coupling means is a length of waveguide structure having substantially the same shape as that of said stationary waveguide means.
 14. A system as in claim 11 in which the ratio of the speed of light to said phase velocity is approximately from 1.01 to 1.05.
 15. Apparatus as in claim 11 including a waveguide structure in said coupling means, said waveguide structure having an effective length which is more than 10 times as long as the wavelength of the microwave energy being transmitted.
 16. Apparatus as in claim 11 in which said unshielded portion of said stationary waveguide means faces generally downwardly, with said shield covering said waveguide means.
 17. Apparatus as in claim 16 including a plurality of support structures for supporting said waveguide means above the ground, and means for fastening said shield to said support structures.
 18. Apparatus as in claim 11 in which said signal coupling means has a partially shielded waveguide structure substantially identical to that of said stationary waveguide means, and means secured to the shield of said coupling means to hold it in a position such that the unshielded portion of the antenna waveguide structure faces that of the first-named waveguide structure.
 19. Apparatus as in claim 18 in which said waveguide structure in said coupling means has a microwave absorber at one end, and microwave detector means at the other end.
 20. In a microwave communication system, a stationary microwave guide comprising a semicylindrical dielectric guide member with an electrically conductive member on the surface of said guide member extending along a diameter of the full cylinder of which said member is a part, means for generating and sending said microwave energy through said guide member in the HE11 mode, a coupler positioned to move approximately parallel to said stationary guide at a distance therefrom to couple microwave signals to and from said stationary guide, said coupler being a waveguide having substantially the same shape as said stationary waveguide.
 21. A system as in claim 20 in which the waveguide of said coupler has an effective length which is more than 10 times as long as the wavelength of the microwave energy being transmitted.
 22. A sYstem as in claim 20 in which said electrically conductive member comprises a shield which covers said dielectric guide member, and including support members fastened to said conductive member to support said guide above a support surface.
 23. A system as in claim 22 in which said shield has, substantially, an inverted U-shape.
 24. In a microwave energy transmission system, elongated waveguide means for guiding microwave electromagnetic energy at a phase velocity less than the speed of light in air, a conductive shield on only a portion of the perimeter of said waveguide means, said shield being substantially perpendicular to the lines of force of the electrical field in said waveguide means when conducting microwave energy in a predetermined mode, a plurality of support structures for supporting said waveguide means above the ground and means for fastening said shield to said support structures, antenna means adjacent the unshielded portion of said waveguide means, said antenna means having a partially shielded waveguide structure substantially identical to that of the transmission system, and means secured to the shield of said antenna means to hold it in a position such that the unshielded portion of the antenna waveguide structure faces that of the first-named waveguide structure, said waveguide structure of said antenna having a length which is an integral multiple of half-wavelengths of the energy being transmitted, a pair of microwave reflectors at the ends of the antenna with an aperture in one reflector, and means for guiding energy out of said antenna through said aperture. 